Coastal erosion is widespread and
locally severe in Hawaii and other low-latitude areas. Typical
erosion rates in Hawaii are in the range of 15 to 30 cm/yr (0.5
to 1 ft/yr; Hwang, 1981; Sea Engineering, Inc., 1988; Makai Ocean
Engineering, Inc. and Sea Engineering, Inc.,1991). Recent studies
on Oahu (Fletcher et al., 1997; Coyne et al., 1996) have shown
that nearly 24%, or 27.5 km (17.1 mi) of an original 115 km (71.6
mi) of sandy shoreline (1940's) has been either significantly
narrowed (17.2 km; 10.7 mi) or lost (10.3 km; 6.4 mi). Nearly
one-quarter of the islands' beaches have been significantly degraded
over the last half-century and all shorelines have been affected
to some degree. Oahu shorelines are by far the most studied,
however, beach loss has been identified on the other islands
as well, with nearly 13 km (8 mi) of beach likely lost due to
shoreline hardening on Maui (Makai Engineering, Inc. and Sea
Engineering, Inc., 1991).
Causes of coastal erosion and beach loss in Hawaii are numerous
but, unfortunately, poorly understood and rarely quantified.
Construction of shoreline protection structures limits coastal
land loss, but does not alleviate beach loss and may actually
accelerate the problem by prohibiting sediment deposition in
front of the structures. Other factors contributing to beach
loss include: a) reduced sediment supply; b) large storms; and,
c) sea-level rise. Reduction in sand supply, either from landward
or seaward (primarily reef) sources, can have a myriad of causes.
Obvious causes such as beach sand mining and emplacement of structures
that interrupt natural sediment transport pathways or prevent
access to backbeach sand deposits, remove sediment from the active
littoral system. More complex issues of sediment supply can be
related to reef health and carbonate production which, in turn,
may be linked to changes in water quality. Second, the accumulated
effect of large storms is to transport sediment beyond the littoral
system. Third, rising sea level leads to a natural landward migration
of the shoreline.
Dramatic examples of coastal erosion, such as houses and roads
falling into the sea, are rare in Hawaii, but the impact of erosion
is still very serious. The signs of erosion are much more subtle
and typically start as a "temporary" hardening structure
designed to mitigate an immediate problem which, eventually,
results in a proliferation of structures along a stretch of coast.
The natural ability of the sandy shoreline to respond to changes
in wave climate is lost.
The overall goals of this study are to document the coastal erosion
history in Hawaii, determine the causal factors of that erosion,
provide high-quality data for other "end-users" in
applied studies (i.e. coastal engineers, planners, and managers),
and increase our general understanding of low-latitude coastal
geologic development. This project involves close cooperation
between the USGS Coastal and Marine Geology Program and the University
Coyne, M.A., Fletcher, C.H.,
and Richmond, B.M. 1999. Mapping Coastal Erosion Hazard Areas
in Hawaii: Observations and Errors. Journal of Coastal Research,
Special Edition 28, p. 171-184.
Fletcher, C.H., Mullane,
R.A., and Richmond, B.M. 1997. Beach Loss along armored shorelines
on Oahu, Hawaiian Islands. Journal of Coastal Research,
13(1), p. 209-215.
Hwang, D.J., 1981. Beach
Changes on Oahu as Revealed by Aerial Photographs. Hawaii
Office of State Planning, Coastal Zone Management Program, Honolulu,
Makai Ocean Engineering, Inc. and Sea Engineering, Inc., 1991.
Aerial Photograph Analysis of Coastal Erosion on the Islands
of Kauai, Molokai, Lanai, Maui and Hawaii. Office of State
Planning, Coastal Zone Management Program, Honolulu, HI., 199p.
Sea Engineering, Inc., 1988. Oahu Shoreline Study. City
and County of Honolulu, Department of Land Utilization, 61p.